Carbonation ash reactivation process and system for combined SOx and NOx removal

ABSTRACT

The present invention includes methods and apparatus useful in the removal of air pollutants. More specifically, this invention relates to methods and apparatus useful in mitigating major air pollutants (SO x  and NO x ) and trace toxins from coal-fired combustors. Using a method or apparatus of the present invention, a coal-fired combustor may be retrofitted to accommodate combined SO x /NO x  removal technology for solid waste reduction and environmentally responsible utilization of dry flue gas (FGD) desulfurization product. The combined SO x /NO x  control technology may integrate enhanced removal of SO 2  at high to medium temperatures using a desulfurization process of the present invention with selective catalytic reduction technology for NO x . The reactivation of spent sorbent and dry FGD product may result in a more complete utilization of the ash and sorbent in the reduction of SO 2  emissions, thereby reducing significantly the amount of sorbent used and the volume of by-product generated. Modifications to a power plant may result in significant changes to the waste stream. Based upon the process for dry FGD product reactivation to achieve enhanced SO 2  removal and SCR for NO x  removal, it may be possible to remove both sulfur and nitrogen oxides from high sulfur coal. The process is modular in nature (different components of the process can be by-passed as and when the need arises) and thus may have great flexibility and be applicable to various operating conditions.

[0001] This application is a Divisional of U.S. application Ser. No.09/363,912, filed Jul. 28, 1999, now U.S. Pat. No. ______, which isincorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention is in the field of pollution andcontaminant removal.

BACKGROUND OF THE INVENTION

[0003] This invention relates to apparatus useful in the removal of airpollutants. More specifically, this invention relates to apparatususeful in mitigating major air pollutants (i.e., SO_(x) and NO_(x)) andtrace toxins from coal-fired combustors.

[0004] The residual solids resulting from various flue gasdesulfurization (FGD) processes, including scrubber sludge, containsignificant portions of unreacted sorbent. Unless these solids aretreated, they will be sent to landfills, thus increasing the costassociated with sorbent requirements and waste disposal.

[0005] Nitrogen oxides (NO_(x)) are emitted when fossil fuels such ascoal, natural gas, or oil are burned in air. NO_(x) emissions haveattracted increased attention in recent years as more is learned abouttheir role in acid rain, smog, visibility impairment and global climatechange. About half of all nationwide NO_(x) pollutants come fromautomobiles, whereas coal-burning utility boilers contribute about 25%of the total. The 1990 Clean Air Act amendments require all coal-firedutility boilers over a certain size to reduce NO_(x) by about 50%. Inaddition, it is expected that regulations affecting the emission ofNO_(x) will get tougher in the future and power plants will need toreduce emissions even further. Another serious problem may occur inintegration, when trace metals and NO_(x) may contaminate the catalyst.

[0006] In coal-fired power plants, disposal of coal combustion productssuch as ashes and wet/dry FGD products is a serious concern. Most ofthese solid wastes are sent to landfills for disposal. Studies haveshown that, when treated properly, these solid waste products can beused beneficially in a number of applications.

[0007] It is therefore an object of the invention to provide acost-effective method and apparatus for reducing the residual solidsproduced during the mitigation of major air pollutants and trace toxinsfrom coal-fired combustors by recycling the unreacted sorbent containedin those solids.

[0008] Although described with respect to the field of mitigating majorair and trace toxins from coal-fired combustors, it will be appreciatedthat similar advantages may obtain in other applications of the presentinvention. Such advantages may become apparent to one of ordinary skillin the art, in light of the present disclosure or through practice ofthe invention.

SUMMARY OF THE INVENTION

[0009] The present invention includes a reactivation technique developedfrom a fundamental understanding of the pore structural properties ofboth CaCO₃ and Ca(OH)₂, and the evolution of pore structure withcalcination and sintering.

[0010] Integration of a SO_(x) removal process of the present inventionwith SCR technology for NO_(x) removal offers an attractive alternativeto post-combustion flue gas clean-up technologies as it not only reducesthe emission of acidic pollutants but also reduces the amount ofsolid-waste generated. The combined SO_(x)/NO_(x) technology of thepresent invention integrates a novel ash reactivation process for SO_(x)removal with proven SCR technology for NO_(x) removal. The coal firedpower plants that use high sulfur coals can be encouraged to continueusing these coals by retrofitting to include the inventive process foradvanced, cost effective NO_(x) and SO₂ removal combined with reducedsolid waste generation and increased dry FGD product utilization.

[0011] A recycling of spent sorbent and fly ash mixture into the spraydryer may result in substantial improvements in reagent utilization andSO₂ removal. Substantial reactions may occur between the fresh Ca(OH)₂and recycled fly ash from spray dryer, resulting in the formation ofhydrated calcium silicates. Their subsequent reaction with SO₂ may leadto increased efficiency. The recycling of used sorbents is described inco-pending application Ser. No. 09/073,237, which is hereby incorporatedherein by reference.

[0012] Although not limited to the theory of the invention, the key tothe high reactivity of a fresh or partially utilized sorbent may lie inits open initial internal structure and subsequent pore structureevolution under high temperature conditions. The present reactivationtechnique may be used for spent and under-utilized sorbents, andbenefits from the pore structural properties of both CaCO₃ and Ca(OH)₂,and the evolution of pore structure with calcination and sintering. Thepresent invention includes a suspension-based carbonation process inwhich the unreacted CaO is converted into calcium carbonate (CaCO₃)instead of calcium hydroxide (Ca(OH)₂).

[0013] Along with reactivation of unreacted CaO, this process provides abetter distribution/exposure of available calcium than the reactivatedspent sorbent from hydration alone. The process of the present inventionhas been successfully applied to the reactivation of two partiallyutilized sorbents generated in the laboratory, and has been furtherdemonstrated to reactivate two commercial ash samples under bench-scaleconditions.

[0014] Accordingly, the present invention includes devices and systemsuseful in removing air pollutants. This invention also includes machinesor instruments using these aspects of the invention. The presentinvention may be used to upgrade or retrofit existing machines orinstruments using methods and components known in the art.

[0015] The present invention also includes methods and processes usingthe devices of the present invention. The methods and processes of thepresent invention may be applied using procedures and protocols knownand used in the arts to which they pertain.

[0016] In broadest terms, the present invention includes a method ofremoving SO_(x) and trace metals from a gaseous waste stream from coalcombustion, where the coal combustion generates an untreated gaseouswaste stream containing SO_(x) and trace metals, and the treatment ofthe gaseous waste stream generates a source of limestone, lime or slakedlime, comprising the steps: (a) admixing carbon dioxide with the sourceof limestone, lime or slaked lime and water so as to carbonate thelimestone, lime or slaked lime, whereby a carbonated sorbent isproduced; and (b) contacting the gaseous waste stream containing SO_(x)and trace metals with the carbonated sorbent, so as to remove SO_(x) andtrace metals from the gaseous waste stream. The water may additionallycontain at least one substance selected from the group consisting ofsurfactants and modifiers. The gaseous waste stream containing SO_(x)and trace metals may be contacted with the carbonated sorbent in acirculating fluidized bed reactor. The gaseous waste stream mayadditionally contain NO_(x) species, and may be contacted with acatalyst adapted to remove the NO_(x) species following step (b). Theflow of clean flue gas containing carbon dioxide may be obtained fromcontacting the gaseous waste stream with a catalyst adapted to removeNO_(x) species following step (b).

[0017] The present invention also includes, in broadest terms, a systemfor removing SO_(x) and trace metals from a gaseous waste stream fromcoal combustion, whereby the coal combustion generates an untreatedgaseous waste stream containing SO_(x) and trace metals, and thetreatment of the gaseous waste stream generates a source of limestone,lime or slaked lime and a flow of clean flue gas containing carbondioxide, comprising: (a) a coal-burning facility producing a source ofan untreated gaseous waste stream containing SO_(x) and trace metals,and an apparatus for removing SO_(x) so as to generate a source oflimestone, lime or slaked lime; (b) a carbonation reaction container foradmixing the flow of said clean flue gas containing carbon dioxide withthe source of limestone, lime or slaked lime with and water so as tocarbonate said limestone, lime or slaked lime, thereby producing acarbonated sorbent; (c) a dryer adapted to remove water from thecarbonated sorbent; and (d) a sorbent reaction container for contactingthe gaseous waste stream containing SO_(x) and trace metals with thecarbonated sorbent, so as to remove SO_(x) and trace metals from thegaseous waste stream.

[0018] The apparatus for removing SO_(x) may be selected from the groupconsisting of wet, dry and wet-dry scrubbers. The system mayadditionally comprise a catalytic reaction container for contacting thegaseous waste stream with a catalyst adapted to remove NO_(x) speciesfollowing treatment in the sorbent reaction container. The catalyticreaction container may comprise a selective catalytic reductioncatalyst. The system may additionally comprise a conduit adapted toconduct a flow of clean flue gas from the sorbent reaction container tothe carbonation reaction container. The system may also comprise aconduit adapted to conduct a flow of clean flue gas from the catalyticreaction container to the carbonation reaction container.

[0019] The system may additionally comprise a conduit adapted to conductthe source of limestone, lime or slaked lime from the coal-burningfacility to the carbonation reaction container. The system may alsocomprise a conduit adapted to conduct the untreated gaseous waste streamfrom the coal-burning facility to the sorbent reaction container. Thesystem may contain a conduit adapted to conduct a flow of water to thecarbonation reaction container. The system may include at least oneparticle separator, and may include a heat exchanger adapted to supplyheat from the flow of clean flue gas to the dryer. The sorbent reactioncontainer may also comprise a circulating fluidized bed reactor.

[0020] The present invention also includes, in broadest terms, a methodof preparing a sorbent from limestone, lime or slaked lime generatedfrom the removal of SO_(x) from a gaseous waste stream from coalcombustion, comprising the steps: obtaining said limestone, lime orslaked lime; and (b) admixing carbon dioxide and water with thelimestone, lime or slaked lime so as to carbonate the limestone, lime orslaked lime, thereby producing a carbonated sorbent.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 shows the hydration reactivation of spent sorbent inaccordance with one embodiment of the present invention.

[0022]FIG. 2 shows the carbonation reactivation of spent sorbent inaccordance with one embodiment of the present invention.

[0023]FIG. 3 is a schematic of a process of the present invention forcombined SO_(x)/NO_(x) removal in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0024] In accordance with the foregoing summary, the following presentsa detailed description of the preferred embodiment of the invention thatis currently considered to be the best mode.

[0025] The method of the present invention may be used as a combinedSO_(x)/NO_(x) removal system, utilizing coal by-products. This method ofthe present invention for reactivating the partially utilized sorbent isbased on a suspension-based carbonation process. The process involvesconverting the unreacted CaO into calcium carbonate (CaCO₃), as shown inFIG. 2, instead of calcium hydroxide (Ca(OH)₂), as shown in FIG. 1. FIG.1 shows a partially sulfated CaO particle 4. Water may seep through theCaSO₄ layer 5 to react with the unsulfated CaO and form high molarvolume Ca(OH)₂. The formation of high molar volume Ca(OH)₂ may lead tothe development of cracks in the CaSO₄ layer 5. FIG. 2 shows theintroduction of CO₂ to the Ca(OH)₂ particle, which may then react toproduce the desired CaCO₃ 7. Combined with reactivation of unreactedCaO, this process may also provide a better distribution/exposure ofavailable calcium than the reactivated spent sorbent from hydrationalone.

[0026] The process flow diagram with accompanying control devices isshown in a schematic given in FIG. 3. Part of the dry FGD product andash 25 from the bag-house is sent to a slurry bubble column 15(carbonator) for hydration and carbonation. A side stream 24 from theclean/scrubbed flue gas 30 may be used to provide CO₂ for carbonation ofthe ash. A water inlet 34 is provided. Following carbonation, the slurryis sent to a continuous filtration unit 16 to reduce the water contentto 35%. Remaining water in the reactivated ash is removed in acontinuous powder dryer 17. Water from the continuous filtration unitmay be cycled through the system or neutralized by addition of dilutealkaline solutions. The effluent water may also be reused as a wettingagent for the wet-dry scrubber or discharged to the sanitary sewer 23.

[0027] Reactivated ash with less than 2% moisture content may then bestored for later use in the reactivated ash storage container 18. Thisreactivated ash, possibly along with limestone from the contingentlimestone storage container 19, is introduced into the fluidized-bedriser reactor 8 as makeup sorbent along with flue gas from the flue gasinlet 22 and recycled solids from the primary particle separator 9 andthe secondary particle separator 10, which pass the ash to the primaryash storage hopper 20 and secondary ash storage hopper 21 respectively.Ash from the storage hoppers may be used in the ash recycle 26 or passedto the ash disposal 27. Hot gases with reduced SO_(x) levels leaving thefluidized-bed riser reactor 8 are heat-exchanged with wet reactivatedash in a powder dryer unit 11 and pass through another particleseparator 12 prior to entering into the SCR reactor 13. Ammonia isintroduced via line 32 upstream of the SCR reactor to ensure effectivemixing. An SCR bypass 31 is also provided. The SCR reactor is designedas a staged fixed-bed reactor with provision for aggressive sootblowing, with 28 as the inlet and 33 as the outlet. Staging of thecatalyst bed is considered necessary to reduce the overall pressure dropand to facilitate effective soot blowing. Gases coming out of the SCRreactor pass through the air heater 14, and through the clean flue gasoutlet 30 and are redirected into the main breaching of the power plantand carried into the low-temperature wet-dry scrubber 33. An air inlet29 is provided for the air heater, and the hot air passing from the airheater 14 is passed on to the powder dryer 17 via heated air line 35.The powder dryer 17 transfers the thermal energy of the heated airstream transported in heated-air line 35 to the reactivated ash so as toreduce the moisture content of the reactivated ash via heat exchanger37. Exhaust line 36 discharges the air from the powder dryer 17 afterthe thermal energy of the heated air stream has been transferred. Theoverall design of the process is modular with inherent flexibility tosuit any operating conditions at operator's discretion.

[0028] Discussion of Results

[0029] Selective Catalytic Reduction (SCR) technology involves thecatalytic reaction of ammonia that is injected into the flue gascontaining NO_(x) to produce nitrogen. Specifically, hot flue gasesleaving the economizer section of the combustor may be directed into acatalytic reactor. Prior to entering the reactor, ammonia may beinjected into the gas stream. Currently, most of the commercialformulations of SCR catalyst comprise vanadia (V₂O₅) as the activematerial deposited on or incorporated with a substrate (TiO₂). Thequantity of ammonia injection may be regulated to provide optimumoperation of the reactor at a temperature of 375° C. with minimumammonia slippage (less than 5 ppm).

[0030] Ca-based sorbents, particularly quicklime (CaO), limestone(CaCO3) and hydrated lime (Ca(OH)₂), are used extensively in FGDprocesses in coal-fired combustors. Currently they all suffer from lowreactivity and sorbent under-utilization. Typically, because of poreblocking and pore mouth plugging, less than 70% of the available calciumis converted to high molar volume calcium sulfate product. As aconsequence, the spent sorbent from a typical FGD process containssignificant amounts of unused sorbent which, unless treated, is disposedof as a solid waste. This in turn may lead to increased costs associatedwith sorbent disposal and need for fresh sorbent. The spent sorbentexhibits negligible reactivity towards SO₂ unless reactivated to exposethe unreacted CaO. Reactivation of the under-utilized sorbent maynecessarily require re-exposing and/or redistribution of the CaO fromthe interior of the sorbent particle and reactivation of the sinteredCaO by converting it into a more reactive form. The fundamentalchallenge and goal of the reactivation process may be to redistributethe CaSO₄ that is located predominantly on the surface of the particleto a more uniform distribution.

[0031] One of the methods for reactivating partially utilized sorbentsis hydration. In this process, the unsulfated CaO is reacted with waterto form Ca(OH)₂. Due to the higher molar volume of the hydroxide (33cc/gmol), compared to CaO (17 cc/gmol), the sorbent particle may expandand the non-porous CaSO₄ shell crack, thereby exposing the hydrate.Reactivation of spent lime/limestone samples from circulating fluidizedbed combustor via hydration may cause particle expansion with anincrease in internal volume. Moreover, once this reactivated sorbent isreintroduced into the combustor, calcination of the Ca(OH)₂ may furtherincrease the porosity and provide added exposure of CaO to SO₂.Hydration has been known to increase the utilization of spent sorbentfrom 35% up to 70%. The above mentioned mechanisms for reactivation ofspent sorbent via hydration suggest that big particles may undergoreactivation by particle expansion and subsequently develop cracks onthe outer inactive sulfate shell. Reactivation of particles that are ofsmaller dimension might be due to reactions between silica/aluminaspecies and calcium leading to the formation of Ca—Si—Al hydratedcomplexes. These complexes have high surface areas and may be highlyeffective for gas-solid reactions.

[0032] Reduction of NO_(x) emissions may be carried out by variousmeans. Modifications or retrofitting during combustion may reduce NO_(x)generation and subsequent emissions by as much as 40%. Combustionmodifications may be limited to simply installing an oxygen meter andreducing the use of excess air; or they may incorporate a morecapital-intensive initiative by installing low NO_(x) burners that mayrequire extensive modification to the furnace and may not beeconomically suitable for small or older units.

[0033] Post-Combustion controls may mean using ammonia with or without acatalyst to remove the NO_(x) in the flue gases. Reduction without thecatalyst may be carried out at higher temperatures and may suffer fromsome serious drawbacks. Reduction in the presence of catalyst orselective catalytic reduction is fast becoming a method of choice forreducing post-combustion NO_(x). This process technology may be appliedto a wide range of coal, oil, and gas fired boilers with demonstratedsuccess in achieving greater than 90% NO_(x) reduction.

[0034] Slip of ammonia is a concern in the application of SCR to acoal-fired combustor as it leads to the formation of ammonium bisulfate(NH₄HSO₄) which may cause severe corrosion problems as it condenses onthe downstream equipment. The formation of NH₄HSO₄ may be directlyrelated to the sulfur content of the coal used. Combustion ofhigh-sulfur coal may lead to formation of higher flue gas SO₂ content,which might cause more SO₂ to be converted to SO₃ in the SCR reactorthereby aggravating the NH₄HSO₄ problems. For effective application ofSCR technology, it may be important for the SO₂ content of the flue gasto be substantially lowered upstream of the SCR reactor.

[0035] Utility coal-fired boilers that have low-temperature FGD (wet orwet-dry scrubbers) processes for reduction of SO₂ emission but arenon-compliant for NO_(x) emission face a daunting task in controllingthe NO_(x) emission while continuing the use of high-sulfur coal. Apossible strategy for control of NO_(x) emissions for such units isintegration of post-combustion SO₂ control and SCR technology. Theinventive process provides a sorbent that may be used for reduction inSO₂ levels upstream of SCR reactor.

[0036] One of the environmentally responsible, yet profitablealternatives for dry FGD product usage is in construction industry.Strength, permeability, and stiffness determinations were made forseveral FGD materials. The values obtained were compared with theengineering properties of conventional construction materials.

[0037] The effectiveness of the inventive ash reactivation process and acomparison of the process with hydration reactivation process wasestablished after conducting extensive studies in a bench-scale set-upwith two in-house generated spent sorbents and two ash samples from acommercial coal combustor.

[0038] SCR is already in use overseas at power plants that burn naturalgas, oil, and low-sulfur coals; however, the technology has never beenfully demonstrated on high-sulfur coals in the U.S. utility market. TheGulf Power Company Plant Crist in Pensacola, Fla. was the site of aClean Coal Technology project sponsored by DOE to demonstrate the use ofSCR technology for NO_(x) reduction. The project demonstrated the use ofSCR technology at high and low dust loading of the flue gas to provide acost-effective means of reducing NO_(x) emissions from the power plantburning high sulfur coal. In this demonstration project, the SCRfacility consisted of three 2.5 MWe equivalent SCR reactors supplied byseparate 5,000 scfm flue gas slipstreams and six smaller 0.2MWe-equivalent SCR reactors. These reactors were calculated to be largeenough to provide design data that would allow the SCR reactors to bescaled-up to the commercial size. Removal of over 80% NO_(x) at ammoniaslip well under 5 ppm was demonstrated.

[0039] In the present combustion configuration for coal-fired boilers,selective catalytic reduction (SCR) of NO_(x) may be the most promisingtechnology for achieving the drastic reduction in NO_(x) levels mandatedby EPA regulations. If the coal used in the facility has high sulfurcontent, however, it may be imperative that SO_(x) removal from flue gasbe undertaken, possibly at higher temperature, prior to removing theNO_(x) by SCR. The present invention, with demonstrated bench-scalesuccess in removing SO_(x), may be integrated with the SCR technology toeffectively reduce the SO_(x) and NO_(x) emissions while reducing theFGD.

[0040] The SCR technology offers the following benefits: it may be oneof the few NO_(x) technologies capable of removing high levels of 80% ormore; it may be applicable to all types of boilers, includingcyclone-fired boilers which cannot be easily retrofitted with othertypes of NO_(x) control technologies; and it may be used with both newand existing power plants. The demonstration of SCR technology may bedesigned to address several uncertainties, including potential catalystdeactivation due to poisoning by trace metal species, performance of thetechnology in the presence of high amounts of SO_(x), and performance ofthe SCR catalyst under typical high sulfur coal operating conditions.

[0041] The presence of arsenic and other trace element species may bedetrimental to the performance of SCR catalyst and is known to causecatalyst deactivation and reduced life. Calcium-based sorbents haveshown good results for capture of trace metallic species. Calcium oxidemay react with both selenium and arsenic to form calcium selenite andarsenate respectively, and thus can be effectively used as a sorbent forthese two trace species at medium temperatures (400-600° C.).Experimental results by other researchers have shown calcium-basedsorbents to be ineffective in lead, cadmium, arsenic, and seleniumremoval from flue gas stream at temperatures in the range of 500-800° C.

[0042] The operation of the inventive desulfurization process upstreamof the SCR reactor may not only lower the SO₂ levels in the flue gasstream, but preserve the SCR catalyst by effectively reducing the tracemetal loading of the flue gas.

[0043] Although most of the inorganic mater in coal remains in the ash,coal combustion does lead to volatilization of some of the low-boilingtrace elements and their subsequent transfer into the gas-phase. Somehigh volatility trace elements are exclusively emitted as vapor throughthe stack. Some of the less volatile trace elements partly deposit onthe ash particles or condense as aerosol particles as the flue gas coolsdown and are partly emitted into the atmosphere as vapor andparticulates. These elements are known as chalcophiles. As, Se, Pb, Sb,and Cd are some examples of elements that display such behavior.

[0044] Most of the chalcophilic elements have been identified as airtoxics from coal-fired combustor and utility boilers in the 1990 CleanAir Act Amendments (CAAA), and are also considered potential SCRcatalyst poisons. The U.S. EPA has been conducting extensive research todetermine their health and environmental effects. The control ofchalcophilic emissions presents a formidable technical and economicalchallenge to the operation of coal-fired boilers. This is due in part tothe lack of understanding of the behavior of these elements and also dueto the fact that they exist in only trace amounts. Considerable efforthas been made in last 4-5 years to determine the speciation of theseelements. Most of the research has concentrated on determining the exactchemical form(s) of these elements in a highly varied and heterogeneousflue gas environment, with very limited attention being focused on theactual methodology to be applied to their control. Application ofsorbents, especially calcium-based sorbents, has shown considerablepromise in irreversibly capturing some of these trace species.

[0045] Methods and Materials

[0046] The following process equipment description represents theprimary component systems of the inventive SO_(x)/NO_(x) process. Theremay be considerable system integration with the scrubber unit orexisting site equipment that could also be utilized.

[0047] Riser Reactor for SO_(x) Control: At high SO₂ levels the catalystin the SCR reactor may oxidize SO₂ to SO₃ which would react with ammoniato give ammonium bisulfate. Therefore, it may be important to remove orlower the levels of SO₂ prior to end the gas to the SCR unit. The SO_(x)reactor may be a fluidized-bed riser reactor. The hot flue gas streamgenerated during coal combustion may be introduced from the bottom ofthe riser part of the reactor to reduce the SO₂ levels by reacting withreactivated ash sorbent upstream of the SCR reactor. The fluidized-bedriser reactor may be designed to provide average solid hold-up of 10%,superficial gas velocities sufficient for fluidization (3-4 m/s), andoptimum mixing and retention times. Sample ports may be added at the keylocations to test for NO_(x), SO_(x), and toxin concentration. Inlet andoutlet compositions of the gas stream may be monitored continuously forpurposes of documenting the performance of the reactor and thereactivated sorbent.

[0048] SCR Reactor: Ammonia may be injected into the gas stream at asufficient distance upstream of the SCR reactor to allow optimum timefor complete mixing. The NO_(x) reactor may be a staged fixed-bedreactor, with plates of catalyst spaced preferably at equal distances of0.5 inches. The flue gas stream may enter the reactor at a temperatureof 375° C. The reactor may be equipped with soot-blowing accessories inthe form of lances placed between the catalyst plates. Currently, mostcommercial formulations of the SCR catalyst use vanadia (V₂O₅) as theactive material deposited on or incorporated with a substrate (TiO₂).The quantity of ammonia injection may be regulated to provide optimumoperation of the reactor at a temperature of 375° C. with minimumammonia slippage (less than 5 ppm). The presence of SO_(x) riser reactorwith calcium based sorbent upstream of the SCR reactor may enhance theperformance of the catalyst by removing the presence of trace heavymetals which are considered to be poisonous to the catalyst.

[0049] Slurry Bubble Column (Carbonator): Reactivation of the dryscrubber products and ash may be carried out in a slurry bubble columnat ambient temperatures. A side stream of clean flue gas may be used toprovide CO₂ for carbonation and agitation. The slurry bubble reactor maybe designed to operate with a solids concentration of 10%. Thecarbonation reactivation of the partially used sorbent and ash mayresult in the re-exposure and redistribution of the unreacted/unutilizedcalcium sorbents for further reaction with SO₂. Continuous monitoring ofthe slurry pH and temperature may then be carried out to determine theefficiency of the carbonation reactivation process.

[0050] Continuous Filtration and Powder Drying: Following thecarbonation reactivation, the slurry may be decanted for filtrationprior to sending the filter cake for drying. The drying of the filercake may be accomplished by contact with the hot gases leaving thefluidized bed reactor. The design of the continuous powder drying may besuch that the gas stream leaving the dryer for the SCR reactor ismaintained at 400° C. The water from the clarifier may be reused in thecarbonator or may be neutralized prior to discharge.

[0051] Control and Analytical Instrumentation: Flue gases from theoverall process may be closely monitored using a Continuous EmissionsMonitor (CEM). Gas composition may be determined on a continual basisupstream of the SO_(x) reactor and downstream of the SCR reactor. Anonline ammonia analyzer may be placed downstream of the SCR reactor toclosely monitor ammonia slippage and to determine catalyst activity. Theammonia delivery system may be equipped with fail-safe control system toterminate the injection of ammonia into the gas stream if the injectedflow rate increases past a pre-determined value. The design of thesystem may provide for gas sampling and temperature monitoring atvarious key locations.

[0052] The preferred embodiments herein disclosed are not intended to beexhaustive or to unnecessarily limit the scope of the invention. Thepreferred embodiments were chosen and described in order to explain theprinciples of the present invention so that others skilled in the artmay practice the invention. Having shown and described preferredembodiments of the present invention, it will be within the ability ofone of ordinary skill in the art to make alterations or modifications tothe present invention, such as through the substitution of equivalentmaterials or structural arrangements, or through the use of equivalentprocess steps, so as to be able to practice the present inventionwithout departing from its spirit as reflected in the appended claims,the text and teaching of which are hereby incorporated by referenceherein. It is the intention, therefore, to limit the invention only asindicated by the scope of the claims and equivalents thereof.

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What is claimed is: Method of Preparing a Sorbent from Limestone, Limeor Slaked Lime Generated from the Removal of SOx from a Gaseous WasteStream from Coal Combustion
 1. A method of preparing a sorbent fromlimestone, lime or slaked lime generated from the removal of SO_(x) froma gaseous waste stream from coal combustion, said method comprising: (a)obtaining said limestone, lime or slaked lime; and (b) admixing carbondioxide and water with said limestone, lime or slaked lime so as tocarbonate said limestone, lime or slaked lime, so as to produce acarbonated sorbent.
 2. A sorbent produced in accordance with the methodof claim 1.